One problem in acoustic and radio frequency communication and measurement systems is receiving a low-level received signal in the presence of a high-level outbound transmitted signal. The outbound signal has a direct path from transmit to receive and may also include some indirect paths such as bouncing off walls, the surface of the ocean, etc. The overall rejection of the outbound signal needs to be around 100 dB for communications and 160-200 dB for some measurement systems.
There are a number of possible techniques for solving these problems. These include isolation which allows for some kind of simultaneous transmission and sensing. Physical cancellation cancels the outbound signal using direct acoustics, direct RF, or analog summing at the front of a Low Noise Amplifier (LNA). Digital cancellation cancelling the outbound signal may use purely digital methods (after an A/D converter).
For acoustic systems, one isolation means possible is the use of separate transmission and reception transducers. Many acoustic communication systems today employ a single transducer which operates for both transmission and reception involving a switch to alternately connect transmission power amplifier and reception LNA. Most acoustic measurement systems already employ separate transducers. For full-duplex communication systems, separate transmission and reception transducers may be possible. The amount of isolation in these systems is proportional to the distance between the transducers squared. One meter typically gives 0 dB of isolation—two meters gives 6 dB of isolation. This is generally insufficient to permit full-duplex operation by itself.
RF isolation is provided by either having separate transmission and reception antennas or by employing a device known as a circulator. A circulator is a 3-terminal device which connects to transmitter power amplifier, antenna and receiver LNA. A circulator typically provides about 20 dB isolation between transmitter and receiver while causing loss of about 1 dB on the transmitter-antenna connection and a similar loss on the antenna-LNA connection.
Some radio systems have sufficient isolation between their two antenna elements to permit full-duplex operation. These systems could still benefit from further cancellation to improve range and sensitivity. Circulators rarely provide sufficient isolation to permit full-duplex operation.
In direct acoustic cancellation, an additional transmission transducer is placed on or near the receiving transducer. A cancellation waveform is played through this transducer to directly cancel the outbound signal. See PCT/US2016/037243 (WO2016/205129) (Judell) incorporated herein by this reference.
In direct RF cancellation, an additional antenna may be coupled to the receiving antenna to directly cancel the outbound signal.
In electronic cancellation methods, an electronic signal is generated and electronically summed into the input of the LNA. This method has been studied for Simultaneous Transmit and Receive (STAR). There are several variants.
Stanford University has successfully employed a method specific to RF communication. They construct a number of delay lines of differing length. Each delay line has a variable attenuator attached to it, with the output of all these attenuators going to a summing junction at the LNA. These provide summed, delayed copies of the transmitted signal for cancellation. The technique, as published, suffers two weaknesses.
The first is a theoretical limit of approximately 50 dB of cancellation caused by coarse quantization of the variable attenuators. The second is that the tuning of the attenuators to obtain that level of cancellation requires switching the system to a separate tuning mode from time to time taking the whole communication network down each time. For practical RF systems, this is likely to be a 10% downtime. For acoustic systems with significant range (1 km to 100 km), this is likely to be a 90% downtime. There is, however, an advantage in directly cancelling power amplifier noise and distortion by 50 dB.
University of Texas, Austin employed a D/A and modulator to provide a cancellation signal to the input of the LNA. Their system obtained approximately 20 dB of broadband cancellation. The conventional wisdom for this type of cancellation is that 20 to 40 dB cancellation is the maximum possible broadband cancellation.
The approach explained in PCT/US/2016/037243 (Judell) enhances the Austin approach. This approach is applicable to both acoustic and RF methods, but specific to the case in which we have full control of the transmission loop. It was demonstrated that by including a delay element in the final transmission stage, Austin-type cancellation can reach arbitrary levels. The longer the delay, the greater the possible cancellation. Cancellation of greater than 100 dB has been demonstrated. This (combined with isolation) can be enough to permit full-duplex operation.
But, this approach may have several weaknesses in some systems.
It cannot cancel transmitter power amplifier noise which is not visible to the cancellation circuitry. The method does cancel amplifier distortion up to the accuracy of the amplifier distortion models used in the canceller. This is adequate for many low-power linear amplifier applications like most RF communication and some high-frequency acoustic systems. Ultimately, cancellation is limited by the dynamic range of the cancellation signal generator. While this doesn't limit the cancellation amount, per se, it does limit the maximum transmitter power. The ratio of cancellation signal generator maximum level to cancellation signal generator noise power spectral density must be 3 dB lower than the ratio of outbound signal power to system noise power spectral density.
There are several applications of Kalman filters to active noise cancellation. See for example, IEEE Signal Processing Letters, Vol. 22, No. 12, December, 2015, “The Random Walk Kalman Filter in Multichannel Active Noise Control,” Paulo A. C. Lopes, Jose A. B. Gerald and Moisés S. Piedad incorporated herein by this reference.
The are notable differences between the new architecture described herein and that described in Lopez et al. For example, Lopez relates to noise cancellation from unintentional, measured sources. These sources are not under control of the system. Therefore, all of the delay and optimization methods disclosed in PCT/US2016/037243 are not applicable. As a result, the Lopez system must be absolutely real-time and cannot deal with non-minimum-phase cancellation paths. The Lopez system, in relying on very short delays, adapts on every new sample of data. There are no long delays anywhere in their system.
For one preferred application, note the following differences. The interfering signal is under our control, insomuch as we can delay it as much as we like. We tend to have large delays between submission of cancellation signals and their response on the receiver. This characteristic of our types of systems means that we absolutely cannot operate on a sample-to-sample basis—the adaptive means would be attempting to update coefficients based on stale cancellation filters. Because of these constraints our adaptation means be able to skip over chunks of data, so that the adaptive algorithm is working strictly on data from the latest adaptive cycle. Adaptation algorithm is preferably capable of working on contiguous chunks of data, corresponding to buffers of digital down converters and/or delta-sigma A/D converters.